Add getLaunchPadUrl() utility function so Launchpad links never break again

[danielbachhuber]

Our 'Launch guide' link on the Sites page was broken for over a month because of #69561. Once we noticed the issue, it was fixed with #70885.

The underlying problem is that manually constructed URLs can be brittle. We should introduce a getLaunchPadUrl() utility function and use it throughout, so Launchpad pad links never break again.

Done is:

• getLaunchpadUrl() is introduced in some appropriate location.
• All links into the Launchpad use the utility function.

[danielbachhuber]

Named routes would be even more awesome: p1670451405142289-slack-C03N25JPCE4

I think a utility function would still be useful in the interim, however.